diff --git a/sha256/sha256block.go b/sha256/sha256block.go
index ca5efd1..44e76b5 100644
--- a/sha256/sha256block.go
+++ b/sha256/sha256block.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !386,!amd64
+// +build !gc
 
 // SHA256 block step.
 // In its own file so that a faster assembly or C version
diff --git a/sha256/sha256block_386.s b/sha256/sha256block_386.s
index 73ae2bf..ad31c25 100644
--- a/sha256/sha256block_386.s
+++ b/sha256/sha256block_386.s
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build gc
+
 // SHA256 block routine. See sha256block.go for Go equivalent.
 //
 // The algorithm is detailed in FIPS 180-4:
diff --git a/sha256/sha256block_amd64.s b/sha256/sha256block_amd64.s
index 868eaed..1d4f7b9 100644
--- a/sha256/sha256block_amd64.s
+++ b/sha256/sha256block_amd64.s
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build gc
+
 #include "textflag.h"
 
 // SHA256 block routine. See sha256block.go for Go equivalent.
diff --git a/sha512/sha512block.go b/sha512/sha512block.go
index 648ae8f..6b1c18c 100644
--- a/sha512/sha512block.go
+++ b/sha512/sha512block.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !amd64
+// +build !gc
 
 // SHA512 block step.
 // In its own file so that a faster assembly or C version
diff --git a/sha512/sha512block_amd64.s b/sha512/sha512block_amd64.s
index 2e10233..12b9eee 100644
--- a/sha512/sha512block_amd64.s
+++ b/sha512/sha512block_amd64.s
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build gc
+
 #include "textflag.h"
 
 // SHA512 block routine. See sha512block.go for Go equivalent.